In the refrigeration cycle, a mechanical device transfers heat from a lower-temperature heat source into a higher-temperature heat sink, whereas heat would naturally flow in the opposite direction. In this cycle, latent heat is released during a liquid to gas phase change. Typically an electric compressor motor is used to drive the refrigeration cycle in which a refrigerant is pumped into a cooled compartment (usually in the form of an evaporator coil), where low pressure causes the refrigerant to evaporate into a vapor, taking heat with it. In another compartment (usually in the form of a the condenser coil), the refrigerant vapor is compressed and forced through another heat exchange coil, condensing into a liquid, and rejecting the heat previously absorbed from the cooled space.
A “heat pump” is a term for a type of air conditioner in which the refrigeration cycle is able to be reversed, thereby producing heat instead of cold in the indoor environment. Using an air conditioner in this way to produce heat is significantly more efficient than electric resistance heating. Some home-owners elect to have a heat pump system installed, which is actually simply a central air conditioner with heat pump functionality (the refrigeration cycle is reversed in the winter). When the heat pump is enabled, the indoor evaporator coil switches roles and becomes the condenser coil, producing heat. The outdoor condenser unit also switches roles to serve as the evaporator, and produces cold air (colder than the ambient outdoor air).
Heat pumps are more popular in milder winter climates where the temperature is frequently in the range of 40-55° F. (4-13° C.), because heat pumps become inefficient below that temperature range. Air source heat pumps (as opposed to geothermal heat pumps) are relatively easy and inexpensive to install, and have therefore historically been the most widely used heat pump type.
However, air source heat pumps suffer limitations due to their use of the outside air as a heat source or sink. Indeed, outdoor condenser coils are ideally situated and installed in full direct sunlight so that the magnitude of the heat source is maximized during colder months. During winter, for example, daytime sunlight advantageously warms the outdoor condenser unit to thereby improve the heat pump's efficiency. Unfortunately, such conditions heat the condenser unit and thereby diminish performance during the summer months when the heat pump is working in cooling mode. Accordingly, a continuing and unmet need exists for new and improved means for improving the cooling performance of heat pumps during the summer.